Don't emit a dead store for '++' operations unless it occurs with a return...

Don't emit a dead store for '++' operations unless it occurs with a return statement.  We've never seen any other cases that were real bugs.

Fixes <rdar://problem/6962292>.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@125419 91177308-0d34-0410-b5e6-96231b3b80d8